1. Field of the Invention
The present invention concerns a structural material for cryogenic temperature use and, more in particular, it relates to a non-magnetic structural material for cryogenic temperature use, required for constituting superconductive magnets. The steel sheet referred to this invention includes steel sheets and steel strips.
2. Description of the Related Art
In various techniques utilizing super conductivity such as nuclear fusion power generation, particle accelerators, and superconductive power storage, superconductive magnets are used in view of the requirement of supplying a large amount of current for generating strong magnetic fields. Since large electromagnetic forces are induced in the superconductive magnet and it is usually cooled to a cryogenic temperature at 2-4 K by liquid helium, structural materials supporting the superconductive magnet require high strength capable of withstanding the large electromagnetic forces under the cryogenic temperature. In addition, since it is a basic object to generate a strong magnetic field at a uniform and stable distribution and in a range as wide as possible, it is essential to minimize the effects of the structural materials on the magnetic fields. Accordingly, it is an essential condition for the materials that they are non-magnetic materials not causing interaction with the magnetic fields.
In view of the above, the structural materials used at the inside or the periphery of the superconductive magnet are required to have high mechanical characteristics and extremely low magnetic permeability at a cryogenic temperature and, further, it is also necessary to take a consideration on thermal deformation in order to firmly hold the superconductive magnet in a composite structure. Further, in the manufacture of the superconductive magnet, it is required for the structural materials that they are excellent in machinability such as punching or boring property or weldability and, further, also excellent in surface flatness or fitness required for laminating a plurality of sheets.
Existent materials considered as structural materials for supporting the superconductive magnet can include austenitic stainless steels, high Mn steels, aluminum alloys, titanium alloys and fiber-reinforced plastics. The mechanical strength, the magnetic permeability and the thermal expansion coefficient required for the structural materials for supporting the superconductive magnet vary depending on the designed intensity of the magnetic fields in the superconductive magnet to be manufactured or the aimed uniformity for the distribution of the magnetic fields and it is important for the selection of the materials that the strength is high, and the permeability and the thermal expansion coefficient are low at a cryogenic temperature.
The fiber-reinforced plastics are non-magnetic and easy to handle with being of low specific gravity and have lower thermal expansion coefficient compared with austenitic stainless steels but the strength per unit cross sectional area is lower. Further, while titanium alloys are low in the specific gravity and high in the strength and have high specific strength, they involve a problem that the toughness is low at a low temperature and is expensive.
Aluminum alloys are used in various applications at cryogenic temperatures since they are light in weight, and have high specific strength and extremely low permeability but they lack in the strength when the designed magnetic fields are applied as in large scale particle accelerators and also involve a problem in the weldability.
Since usual austenitic stainless steels are insufficient in the strength and the toughness at low temperatures, stainless steels of low carbon content with addition of nitrogen have been developed. However, since the stability in the austenitic phase is insufficient in such stainless steels, a portion of the austenitic phase is transformed into a ferromagnetic martensitic phase by deformation at a low temperature. Accordingly, this results in lowering of the toughness and involves a problem that the permeability can not be lowered sufficiently at a cryogenic temperature.
Subsequently, austenitic stainless steels with further increased Ni. content have been developed but they involve a problem of increased cost and high thermal expansion coefficient as the structural material for cryogenic temperature use.
In view of the problems described above, Japanese Patent Publications No. 11661/1984 and No. 18887/1993 propose relatively inexpensive high Mn non-magnetic steels and manufacturing methods thereof. However, the high Mn non-magnetic steels described in Japanese Patent Publication No. 11661/1984 have high permeability at a cryogenic temperature and involve problems as a large scale particle accelerator use. The technique disclosed in Japanese Patent Publication No. 18887/1993 involves problems of requiring long time aging treatment and lowering the productivity.
Further, in the superconductive magnet, a non-magnetic member referred to as a collar is required as fixing members for superconducting wires as conductor coils and the collar is formed by laminating a plurality of non-magnetic steel sheets. Then, the collar also requires an appropriate mechanical strength in order to withstand strong electromagnetic forces caused when it is cooled to a cryogenic temperature and a large amount of current is supplied as the superconductive magnet. However, when the mechanical strength of the non-magnetic steel sheet is excessively high or the residual stress therein is excessive, the working life of a punching die is shortened or warps are caused after punching the non-magnetic steel sheet into a predetermined shape of the collar.
In the superconductive magnet, the collar is often manufactured by precision punching such as fine blanking. With the view point as described above, the mechanical strength of the material used for the collar is determined while taking the strength and the distribution of the designed magnetic field into a consideration. Accordingly, it has been demanded for a method of manufacturing a non-magnetic steel sheet that can easily control the strength of the non-magnetic steel sheet as the material to a desired strength demanded in the design.
An object of this invention is to effectively overcome the foregoing problems in the prior art and provide a method of manufacturing a high Mn non-magnetic steel sheet for cryogenic temperature use, capable of manufacturing, with industrial stability and high productivity, and a high Mn non-magnetic steel sheet which is suitable for use in large scale particle accelerators, and has a high yield point at a cryogenic temperature and low permeability at the cryogenic temperature.
In order to attain the foregoing subject, the present inventors have investigated characteristics required for supporting structural members used in superconductive magnets for use in large scale particle accelerators and have made an earnest study for the factors giving effects on the permeability and the yield stress at a cryogenic temperature of high Mn non-magnetic steel sheets. As a result, it has been found that the permeability of the high Mn non-magnetic steel at the cryogenic temperature can be lowered by further stabilizing the austenitic phase by increasing the content of Mn. Further, it has been found that the yield stress of the high Mn non-magnetic steel at the cryogenic temperature can be controlled easily to 900 MPa or more by applying temper rolling to a steel sheet after intermediate annealing.
This invention has been constituted based on the findings described above. That is, this invention provides a method of manufacturing a hot rolled high Mn non-magnetic steel sheet for cryogenic temperature use, which comprises:
heating a steel material containing, on the weight percent basis:
from 0.05 to 0.18% of C,
from 26.0 to 30.0% of Mn,
from 5.0 to 10.0% of Cr,
from 0.05 to 0.15% of N,
from 0.01 to 0.07% of Al,
from 0.01 to 0.1% of V,
from 0.1 to 1.0% of Si and
from 0.003 to 0.02% of Ca
and hot rolling the material into a hot rolled steel sheet, in which a hot rolling start temperature is from 1050 to 1200xc2x0 C. and the rolling end temperature is 700 to 1000xc2x0 C. for the hot rolling. Further, in a preferred embodiment of this invention, the steel material preferably contains, on the weight percent basis: from 0.05 to 0.18% of C, from 26.0 to 30.0% of Mn, from 5.0 to 10.0% of Cr, from 0.50 to 5.0 of Ni and from 0.05 to 0.15% of N, from 0.01 to 0.07% of Al, from 0.01 to 0.1% of V, from 0.1 to 1.0% of Si and from 0.003 to 0.02% of Ca.
Further, this invention provides a method of manufacturing a cold rolled high Mn non-magnetic steel sheet for cryogenic temperature use, which comprises:
heating a steel material containing, on the weight percent basis:
from 0.05 to 0.18% of C,
from 26.0 to 30.0% of Mn,
from 5.0 to 10.0% of Cr,
from 0.05 to 0.15% of N,
from 0.01 to 0.07% of Al,
from 0.01 to 0.1% of V,
from 0.1 to 1.0 of Si and
from 0.003 to 0.02% of Ca
and hot rolling the material into a hot rolled steel sheet, applying hot rolled plate annealing for the hot rolled sheet then applying cold rolling to form a cold rolled sheet and then applying annealing to the cold rolled sheet, in which a hot rolling start temperature is from 1050 to 1200xc2x0 C. and a rolling end temperature is 700 to 1000xc2x0 C. for the hot rolling and, further, the annealing temperature for the cold rolled sheet annealing is from 1050 to 1200xc2x0 C.
Further, in this invention, the steel material is, preferably, a steel material containing, on the weight percent basis, from 0.05 to 0.18% of C, from 26.0 to 30.0% of Mn, from 5.0 to 10.0% of Cr, from 0.50 to 5.0 of Ni and from 0.05 to 0.15% of N, from 0.01 to 0.07% of Al, from 0.01 to 1.0% of V, from 0.1 to 1.0% of Si and from 0.003 to 0.02% of Ca. Further, in this invention, temper rolling at a draft ratio of 30% or lower is preferably applied after the cold rolled sheet annealing.
At first, the reason for defining the chemical compositions of the steel material is to be explained.
C: 0.05-0.18%, N: 0.05-0.15%
Both of C and N are interstitial solute elements, which are effective for increasing the strength of steels by solid-solution hardening. For obtaining a desired yield stress at a cryogenic temperature, it is necessary to contain C and N by 0.05% or more. On the other hand, when C exceeds 0.18%, the austenitic phase becomes instable to precipitate carbides, and the permeability can no more be kept lower at a cryogenic temperature, and the weldability and the workability are deteriorated. Accordingly, C is defined within a range from 0.05 to 0.18%. A preferred range for C is from 0.07 to 0.15%.
Further, N is an addition element useful for stabilizing the austenitic phase and increasing of the strength at a cryogenic temperature but, if the content exceeds 0.15%, the weldability is deteriorated and abrasion of a tool upon punching fabrication is accelerated, as well as the permeability is increased by precipitation of nitrides or carbonitrides. Accordingly, N is defined within a range from 0.05 to 0.15%. A preferred range for N is from 0.07 to 0.13%.
Mn: 26.0-30.0%
Mn is an important element in this invention, which is useful for stabilizing the austenitic phase and attaining an extremely low permeability even at a cryogenic temperature. In order to obtain such an effect, it is necessary to contain Mn by 26.0% or more. On the other hand, if it exceeds 30.0%, the toughness and the weldability, as well as the productivity are deteriorated, so that Mn is defined within a range from 26.0 to 30.0%.
Cr: 5.0-10.0%
Cr contributes to the increase of the strength by solid-solution hardening and also functions effectively to the improvement of corrosion resistance. Such an effect is recognized at the content of 5.0% or more but the content in excess of 10.0% hinders stabilization of the austenitic phase and results in increase of the permeability at a low temperature. Therefore, Cr is defined within a range from 5.0 to 10.0%. The circumstance in which the material as a target of this invention is used is basically at cryogenic temperature and in high vacuum where chemical reactions proceed extremely slowly, which is not so severe in view of corrosion and a sufficient corrosion resistance can be ensured by the Cr content at such a level. A preferred range for Cr is from 6 to 8%.
Ni: 0.50-5.0%
Ni contributes to the stabilization of the austenitic phase and improvement of the toughness at a cryogenic temperature, as well as improves the corrosion resistance. It can be contained optionally in this invention. Such effect is recognizable at the content of at least 0.50% or more, a great amount of content is not industrially desirable since Ni is expensive. Therefore, Ni is preferably within a range from 0.50 to 5.0%. According to this, the steel material of this invention have remarkable advantages not only in the thermal expansion coefficient but also in view of the cost, as compared with high Ni austenite stainless steels such as SUS 316LN.
Al: 0.01-0.07%
Al is an element effective as a deoxidizer and is a ferrite stabilizing element. If included excessively, it makes austenite phase unstable and consequently an extremely low magnetic permeability, which is an object of the present invention, can not be achieved. But in contrast, when Al content is small, impurities, such as Al2O3 are generated, hot workability is deteriorated, and thus, causes the generation of surface defects originated in cracks. This disadvantageously necessitates treatments such as surface grinding, resulting in decrease in product yield and increase in burdening of load in production processes. Therefore, the lowest Al content is limited to 0.01%.
V: 0.01-0.1% V
An excess amount of V over 0.1% segregates V carbide and V nitride to lessen toughness or lower workability. Also, the area surrounding carbides- or nitride-segregated portion runs short of austenite formers, C and N, and the austenite layer becomes unstable and extremely low magnetic permeability at a cryogenic temperature is difficult to achieve. Although V content should be as small as possible, too small an amount results in conspicuous embrittlement at low temperature and therefore, the lowest limit of V is defined as 0.01%.
Si: 0.1-1.0%
Si is effective as a deoxidizer and can be added as necessary because Si does not deteriorate hot workability as Al does. However, Si, like Al, is a ferrite stabilizing element and if included excessively, austenite phase becomes unstable and consequently, an object of the present invention of extremely low permeability is difficult to achieve. Too small an amount of Si results in insufficient deoxidation, therefore, the lower limit of Si is 0.05, preferably 0.1%.
Ca: 0.003-0.02%
Ca is effective in making S, mingled as an inevitable impurity, harmless and improving hot workability. Such an effect is not secured with a small Ca amount and as such, the lowest Ca content is limited to 0.003%. A preferred addition amount for S is within a range from 0.004 to 0.01%, and it is effective for ensuring the hot workability to satisfy the following equation (1)
0.8xc3x97Ca+30 greater than S+Oxe2x80x83xe2x80x83(1) 
in which the content for each of elements Ca, S and O is indicated on the weight ppm basis. As a more simple criterion for judgement, Ca/Sxe2x89xa72, preferably, Ca/Sxe2x89xa73 may also be used.
The balance other than the chemical compositions described above substantially comprises Fe and inevitable impurities. As the inevitable impurities, S: 0.005% or less, P: 0.05% or less and O: 0.005% or less are permissible with a view point of the industrial economy. Further, it is desirable that the contents for precipitates such as carbides, nitrides and carbonitrides, particularly, Fe3C and Fe4N that form ferromagnetic precipitates or deteriorate the stability of the austenitic phase are as low as possible.
In the method of manufacturing the high Mn non-magnetic steel sheet of this invention, the steel material of the chemical composition as described above is at first heated and hot rolled into a hot rolled sheet.
Since the steel material suitable to this invention contains a great amount of Mn and Mn is easily oxidized at a high temperature, it is not desired to excessively increase the temperature for heating slabs since this not only increases scale losses but also results in excessive formation of Mn fumes. Further, the hot workability of the steel material of the chemical composition described above is not always excellent.